The invention concerns a method and equipment for the reception of digital signals in the form of impulses consisting of bursts of oscillations. With the method, the received signals are first filtered and amplified and are then taken to a signal detector followed by a noise suppression circuit, whereby the signal detector generates an output signal after reception of a signal. It is presupposed that in order to achieve automatic gain control, data and noise signals charge a capacitor in such a way that, when few signals exceed a given threshold level and the charge is correspondingly low, the sensitivity increases, and when many signals exceed the said threshold level and the charge is correspondingly greater, the sensitivity is reduced.
The receiver element of the type U 2506 B from AEG Telefunken is a known circuit which is available on the market in monolithic integrated form. The digital signals to be received are impulses in the form of bursts of oscillations which are modulated using Pulse Position Modulation, and which are taken to an input stage. The signals are then amplified and pass through a high pass filter, with which strong suppression of low frequency noise signals is achieved. After repeated amplification the signals are taken to a low pass filter and are then amplified again. The signals, greatly amplified in this way, are connected to a pulse counter circuit. This consists basically of three comparators connected one after the other, and generates a pulse after reception of a minimum number of oscillations. This pulse is transmitted to a pulse former stage, which as a result generates an output signal of constant duration that represents the useful data signal and whose position in time corresponds to the received signal.
Reception is made insensitive to noise by reducing the amplification in a noise suppression circuit and a circuit for automatic gain control (AGC) in the aforementioned apparatus. With this automatic gain control, the sensitivity rises when few of the received signals exceed a threshold level, but with increasing sensitivity more noise signals come to effect so that the sensitivity must be reduced again. In this way the noise level at the input of the pulse counter stage is kept virtually constant and the sensitivity is optimally adjusted.
As the control criterion with the receiver element described above is a constant interference or noise signal, large quantities of data over a long period of time cannot be transmitted. The receiver element interprets a useful signal in the same way as a noise signal, and therefore during reception of the useful signal the effect of the control is to reduce its sensitivity so much that the data signal can no longer generate the output signal with certainty. As the pulse former stage generates an output signal of constant duration for each received burst of oscillations irrespective of its length, Pulse Burst Width Modulation cannot be used.